1. Field of the Invention
This invention relates to functional fluid compositions, particularly hydraulic fluids and to monoepoxycyclohexyl compounds which are effective to inhibit acid buildup in such fluids.
2. Description of the Prior Art
Functional fluids have been utilized in many different types of applications such as electronic coolants, diffusion pump fluids, lubricants, damping fluids, bases for greases, power transmission and hydraulic fluids, heat transfer fluids, heat pump fluids, refrigeration equipment fluids and as filter mediums for air-conditioning systems. Of these uses, hydraulic fluids intended for use in the hydraulic system of aircraft for operating various mechanisms and aircraft control systems must meet stringent functional and use requirements. One of the most important requirements for an aircraft hydraulic fluid is that the fluid be chemically stable to resist oxidative and thermal degradation which can result in the formation of acids and the corrosive attack of metals in contact with the hydraulic fluid.
In order to control the degree of acid buildup during use of the fluid and inhibit corrosion of the components in the hydraulic system, it is conventional to add certain acid scavengers and/or corrosion inhibitors to the hydraulic fluid base stock.
Although a variety of compounds have been suggested for use as corrosion inhibitors, acid acceptors which act as proton acceptors and prevent the buildup of corrosive acids in the fluids when they undergo decomposition under prolonged use at high temperatures are generally preferred. A particularly preferred class of such materials comprises epoxy compounds, especially epoxidized naturally occurring materials such as epoxidized unsaturated glycerides including epoxidized soybean oil, epoxidized castor oil, epoxidized linseed oil, epoxidized fats and the like. Other suggested materials include epoxy esters such as butylepoxyacetoxystearate, glyceryl triepoxyacetoxystearate, isooctylepoxystearate, epoxidized isooctyl phthalate and the like. Also suggested are various alkyl and arylalkyl epoxides such as epoxy decane, epoxy hexadecane, epoxy octadecane, cyclododecane, and the like, and glyceryl and various glycidyl ethers such as phenyl glycidyl ether, glycidyl cyclohexyl ether, alkyl glycidyl ether, and the like.
More recently it has been suggested that a particular class of epoxy compounds, the 3,4-epoxycycloalkyl-3,4-epoxycycloalkyl carboxylates, are particularly useful as acid acceptors for hydraulic fluids and are more effective than the epoxy compounds used heretofore. A particularly preferred compound is 3,4-epoxycyclohexylmethyl-3,4-epoxycyclohexane carboxylate. These compounds are well known chemical entities which have been used as acid scavengers for chlorinated diphenyl dielectric fluids prior to their introduction as inhibitors for hydraulic fluids.
Although 3,4-epoxycycloalkyl-3,4-epoxycycloalkyl carboxylates are effective acid scavengers for common hydraulic fluid compositions, they have a disadvantage in that they cause resinous deposits to form around the fluid pump shaft at the point of seal. The formation of deposits is of particular concern in aircraft hydraulic systems which operate under pressure and where the deposits soon result in fluid leakage through the seal. Although the problem of shaft seal leakage is not serious from an aircraft operational point of view, it represents a sufficient nuisance that the aircraft industry and aircraft hydraulic fluid manufacturers have actively sought alternative acid acceptors which are as effective and efficient as the 3,4-epoxycyclohexylmethyl-3,4-epoxycyclohexane carboxylate but which do not have the deposit and leakage problem associated with this material.
It is accordingly an object of this invention to provide an acid acceptor effective to prevent acid buildup in functional fluid compositions. Another object of this invention is to provide an acid acceptor which can be used without adverse secondary effects in functional fluids which may also contain a polymeric viscosity index improver. A further object of this invention is to provide functional fluid compositions which are resistant to thermal and oxidative degradation and which are suitable for use in aircraft hydraulic systems. It is a yet further object of this invention to provide an aircraft hydraulic fluid containing a polymeric viscosity index improver and an epoxide acid acceptor which does not cause pump shaft seal leakage. Yet further objects will be apparent from the following description of the invention.